The predominant material used for shipping containers worldwide is corrugated containerboard. Smaller packaging may be formed from a heavy paperboard that may be formed from bleached stock or, more commonly, from bleached or recycled fiber with a white or colored layer laid on one or both surfaces.
Corrugated containerboard is typically formed from two outer or liner plies adhesively bonded to an inner corrugated ply. The overwhelming majority of these containers use an unbleached kraft process fiber for the liner plies. A smaller number are made with a white or mottled white secondary surface on the outer ply or a fully bleached outer ply. Some corrugated containers are also formed by laminating a pre-printed white paper label over some or all of the outer ply. The so-called white or mottled white liner is made by laying down a thin surface of bleached fiber over the unbleached fiber during the papermaking process.
Because of the brown color of the unbleached kraft board, it does not provide a background amenable to attractive color printing. What printing is done on this so-called brownboard has almost universally used opaque inks, black being the most common ink. Opaque inks are also normally used on white or mottled white container-board surfaces although, very infrequently, three color overprinted CMY or four color CMYK (cyan, magenta, yellow, black) transparent inks have been applied using ink jet or other printing technology, such as flexography. These colors are known as “process primaries” and form the basis for most color printing using transparent inks. Shipping container stock is usually printed after the corrugated board is formed. However, the outer liner may also be preprinted before lamination to the single faced board at the double backer station of the corrugating machine.
Colors formed by opaque inks are generally limited to those applied whereas transparent inks laid over top of one another on a white or other color substrate can generate a wide range of new colors. Everybody who has used blue and yellow marker pens for highlighting portions of documents is aware that they form green if overlapped. The presumed reason that transparent inks have not been used on brown-board is that the background color unavoidably changes and degrades the perceived applied color after printing. Color images have been further limited because with opaque inks, depending on the degree of opacity, the uppermost color printed will either significantly or completely obscure anything underlying. Conventional halftone color imaging using opaque inks is used routinely on white stock but this technology is not used on an unbleached substrate.
The present invention goes against the conventional printing wisdom and teaches a method that will produce attractive and reasonably accurate color images on a variety of packaging container stock substrates using transparent inks.